I read it well over 15 years ago, around when I was 10, but don't know how old the book was back then.
There are 2 moments in the book that I remember:
The grandmother (or an elderly woman that the protagonist lived with) was sleep walking with her hands crossed over her chest, palms pointed outwards.
The protagonist has what they think is a dream and opens the curtains in his/her bedroom, revealing that something (a vampire presumably) was looking at them.
I think the grandmother in the above described pose was also on the cover.
Edit: I originally read the book in Dutch, and it is entirely possible that this was the original language.
